msm: vidc: Change encoder default fps to 15

Venus encoder's default fps is 15.  By having the wrong default FPS
within the driver, if client requests for 30 fps, the driver omits
informing venus as it assumes that Venus is already at 30 fps.

Change-Id: Icba465612dbb7df853c9b1092fe84eb35c09368c
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>